I think this is off-topic on all technical SEs, for one simple reason: it’s not about a specific problem you encountered, it’s about a general problem you believe you may encounter.
There are questions related to your situation which would be appropriate on Unix.SE; for example
I tried to upgrade from Fedora Core 29 to Fedora Core 30 and ran into this specific problem (with details). How should I go about fixing it?
That would also be appropriate as a Fedora bug report, and I think it would be more useful as such.
I’m looking for recommendations of operating systems which are upgradable, because I believe I can’t upgrade from Fedora Core 29 to Fedora Core 30, and I’m fed up of re-installing.
isn’t appropriate, because it’s unanswerable: readers can’t guess what the actual concerns you’re trying to address are, so even if this weren’t opinion-based, answerers would have no way of knowing whether their recommendations meets your requirements (because they wouldn’t know why, in your specific circumstances, the upgrade might or might not fail, and therefore whether their recommendation would suffer from the same problem or not).
There is perhaps another variant which might suit your purposes better, and could be on-topic:
I read somewhere (with a link) that in these specific circumstances, upgrading from Fedora Core 22 to 23 fails. Is this still relevant for upgrades from Fedora Core 29 to 30, and if so, is there any way to work around it?